The present invention relates to an improved sealing arrangement for an encapsulated measuring system for measuring the relative position of two objects movable relative to one another, of the type which comprises a housing adapted to be mounted to one of the objects, which housing defines a slit extending in the measuring direction, a measuring component mounted for movement within the housing, and a coupling member mounted to the measuring component to extend out of the slit and adapted to be mounted to the other of the two objects.
In general, in encapsulated measuring systems a longitudinal slit is provided in the housing of the measuring system, and a coupling member extends through this longitudinal slit into the interior of the housing in order to connect a measuring component within the housing such as a scanning unit with a mounting element arranged outside the housing for mounting the coupling member and thereby the measuring component to one of the two objects to be measured. For example, the two objects to be measured can take the form of the bed and the slide piece of a processing machine. In this case, relative movement between the bed and slide piece of the processing machine is transferred to the measuring graduation mounted within the housing of the measuring system or to the scanning unit mounted within the housing of the measuring system. The coupling member is moved in the direction of movement along the length of the slit, thereby causing the measuring component coupled to the coupling member to move in accordance with the movement being measured.
Typically, it is important to protect the highly sensitive measuring graduation included in the measuring system and for this reason the slit in the housing is sealed as closely as possible. Typically, it is in the region where the coupling member passes through the slit that sealing problems are particularly acute and contaminants are most likely to pass into the interior of the housing.
A measuring system of the type described above is shown, for example, in German Patent DE-PS 28 46 768. In this measuring system a scale and a scanning unit are mounted within a housing which defines a slit oriented in the measuring direction. This slit is closed by means of sealing elements which are arranged to meet at an acute angle. These sealing elements take the form of synthetic material or rubber lips, through which the coupling member passes in order to connect the scanning unit with one of the two objects to be measured.
In this measuring system the sealing lips preferably press closely against the coupling member, and are therefore constructed of a resilient elastic material which provides a high restoring force in order to obtain the most hermetic sealing possible of the housing. These high restoring forces can result in high frictional forces between the sealing lips and the coupling member when the coupling member moves along the measuring direction. These high frictional forces can cause deformations of the coupling member in the measuring direction which are, of course, detrimental to measuring accuracy. Furthermore, the bending open of the sealing lips by the coupling member results in a twisting of the sealing lips along the longitudinal extent of the slit. This twisting causes the formation of small zones at the ends of the coupling member which are not entirely sealed by the sealing lips, even when the coupling member is provided with a convex, sword-shaped cross section.
German Patent DE-PS 24 60 406 for this reason suggests the use of a magnetic fluid in such a measuring system in order to seal the gaps between the sealing elements or lips. A magnetic field is used to retain the magnetic fluid in the gaps between the sealing elements in order to provide a more nearly hermetic seal.
The sealing elements described above have been selected as particularly relevant prior art approaches from a large number of publications. These sealing elements can provide an adequate degree of sealing for most applications. However, since the sealing elements described above are made essentially of elastic synthetic material or rubber lips, they do not provide adequate protection against damage to or destruction of the sealing lips by mechanical action.
Measuring systems are often used in conjunction with shaving or chipping machine tools such as milling machines for example. Precisely in the case of such uses hot metallic chips are present near the measuring system, and these hot metal chips may come into contact with the synthetic material sealing lips and destroy them.